Ceramic Matrix Composites (CMS's) are materials that include ceramic fibers embedded in a ceramic matrix. CMC's typically exhibit desirable mechanical, chemical and physical properties at high temperatures. For example, CMS's are typically more resistant to oxidation at high temperatures than are metals. CMC's are generally tougher than monolithic ceramics and exhibit damage tolerance. SiC/SiC CMC's are one example of a composite material that exhibits excellent high temperature mechanical, physical and chemical properties. Such materials are suitable for a number of high temperature applications, such as use in producing hot sector components of gas turbine engines. SiC/SiC CMC engine components allow gas turbine engines to operate at much higher temperatures than engines having superalloy metal components.
The mechanical and thermal properties (such as creep, fatigue strength, elastic modulus, etc.) of the matrix and fiber components of CMC's, and the relationship between those properties, can affect their performance. For example, like other materials, CMC's creep under stress. The relative rates of creep between the matrix and the fibers affect the way in which the CMC bears loads to which it is subjected. For example, if the matrix and fibers exhibit the same amount of creep, they will share the load in the same ratio. However, if the fiber is more creep resistant than the matrix, the matrix will begin to creep first and to a greater extent, which results in the load shifting from the matrix to the fiber. This is acceptable in certain applications because the fiber strength is significantly higher than that of the matrix. For example, depending on the fiber and matrix, the fiber may be approximately 4 to 10 times stronger than the matrix. The differential creep rates also result in a higher matrix cracking stress.
CMC's may be produced by a variety of processes. One process for producing CMC's uses chemical vapor infiltration (CVI) to deposit the matrix material on a network of fibers.